Wireless communication systems often employ power amplifiers. A conventional optimum power amplifier has a low level of DC power consumption and a high level of power added efficiency (i.e. ratio of the difference between the output power and the input power to DC power). In general, a power amplifier operates at maximum power efficiency when the power amplifier transmits peak output power. However, power efficiency worsens as output power decreases.
One type of a power amplifier that addresses the problem of efficiency at lower output powers is a Doherty amplifier. For an overview of the Doherty amplifier, see Doherty, W. H., A New High Efficiency Power Amplifier For Modulated Waves, Proceedings of the Institute of Radio Engineers, Vol. 24, No. 9, pp. 1163-82, September 1936, which is hereby incorporated by reference.
Referring to FIG. 1, a typical Doherty amplifier 100 can include a main amplifier device 102 and an auxiliary amplifier device 104 connected in parallel. An input signal 110 can be split into two signals by an input splitter 112. One of the resulting signals can be coupled to an input of the main amplifier device 102, and another signal can be coupled to an input of the auxiliary amplifier device 104. The output of the main amplifier device 102 can be power combined with the output of the auxiliary amplifier device 104 through a transmission line 106.
The main amplifier device 102 can be configured to be on for the entire range of output powers of the power amplifier. The auxiliary amplifier device 104 can be configured to turn on only when the main amplifier device 102 saturates. The output power at which the auxiliary amplifier device 104 turns on (and the main amplifier device 102 saturates) is referred to as a power transition point PT. The power transition point PT can be determined by a design parameter γ. The design parameter γ is a ratio of the maximum current through the load 108 to the maximum current delivered by the main amplifier device 102.
The transmission line 106 that operates to combine the output of the main amplifier device 102 with the output of the auxiliary amplifier device 104 can be a quarter-wave impedance inverter. The quarter-wave impedance inverter can add a 90° phase lag to the output (e.g., current) of the main amplifier device 102. The phase of the auxiliary amplifier output current is typically designed to lag the main amplifier current by 90° so that the two currents add in phase when the two currents are combined at the output of the impedance inverter.
Referring to FIG. 2, the phase of the output signal of the main amplifier device 102, as a function of input power, (as shown in plot 202) is typically different from the phase of the output signal of the auxiliary amplifier device 104, as a function of input power (as shown in plot 204). The phase of the output signal of the main amplifier device 102 typically decreases (or lags) by a small amount as the input power increases from very low or no input power to the power transition point PT. As the input power further increases beyond the power transition point PT, where the main amplifier device 102 saturates and the auxiliary amplifier device 104 turns on, the phase of the output signal of the main amplifier device 102 typically starts increasing (or leading).
By contrast, the phase of the output signal of the auxiliary amplifier device 104 typically increases as the input power increases from very low or no input power to the power transition point PT. As the input power further increases beyond the power transition point PT, the phase of the output signal of the auxiliary amplifier device 104 typically starts decreasing.
The output power at which both the main amplifier device 102 and the auxiliary amplifier device 104 saturate is the maximum output power PSAT of a typical Doherty amplifier 100. The maximum output power PSAT of the Doherty amplifier 100 can be optimized if the Doherty amplifier 100 is configured such that the output signal of the main amplifier device 102 is substantially in-phase (i.e., has substantially the same phase) with the output signal of the auxiliary amplifier device 104 at output powers close to the maximum output power PSAT. For example, the adjustment in the phases of the output signals of the main amplifier device 102 and the auxiliary amplifier device 104 can be implemented using a fixed phase adjustment network on either the main amplifier device 102 or the auxiliary amplifier device 104.
Nonetheless, as shown in FIG. 2, the output signal of the main amplifier device 102 can still be out of phase with the output signal of the auxiliary amplifier device 104 at output power levels below the maximum output power PSAT and above the power transition point PT. The divergence in the phases of the output signals of the main amplifier device 102 and the auxiliary amplifier device 104 can result in several problems. At output power levels below PSAT and above PT, the currents from the main amplifier device 102 and the auxiliary amplifier device 104 can add out of phase, which can result in low output power and degraded efficiency. Furthermore at output power levels below PSAT and above PT, the phase of the output current (i.e., the phasor sum of the current in the main amplifier device 102 and the current in the auxiliary amplifier device 104) can change significantly with changes in input power, resulting in large AM/PM variations in the Doherty amplifier 100. Large AM/PM variations in the Doherty amplifier 100 can result in increased distortion. Larger AM/PM variations in the Doherty amplifier 100 can also result in a reduced phase margin if the Doherty amplifier 100 is used in a feedback system.